Conventionally, this type of motor includes a worm shaft, which is coaxially coupled to a rotary shaft, or a drive shaft, of a motor body to rotate integrally with the drive shaft. For example, a motor disclosed in Japanese Laid-Open Patent Publication No. 2009-11077 includes a rotary shaft and a worm shaft, which are made of metal. The worm shaft includes a coupling portion formed at an axial end. A coupling hole is formed in the coupling portion. A coupling projection at a distal end of the rotary shaft is inserted into the coupling hole, so that the rotary shaft and the worm shaft are directly coupled to each other.
In the above described motor, for example, when the rotary shaft and the speed reduction mechanism are driven to rotate in the forward direction, the load from the output portion may stop, or locks, the rotation. At this time, the teeth of the worm shaft, or the worm portion, may bind with the teeth of the worm wheel. If the rotary shaft and the speed reduction mechanism begin rotating in the reverse direction from this state, torque for undoing the bound state is required in addition to the torque for rotating the worm shaft.
To solve this problem, clearances may be provided between the inner surface of the coupling hole of the worm shaft and the coupling projection of the rotary shaft both in the circumferential direction and in the radial direction. In a case in which such clearances are provided, when the rotary shaft starts rotating in the reverse direction from the bound state, that is, when the rotary shaft starts rotating in the reverse direction, the rotary shaft is rotated freely for a certain period of time. In this period, the rotary shaft gains an inertial force, or a rotational momentum. After rotating freely due to the clearances, the rotary shaft violently strikes the inner surface of the worm shaft coupling hole with the inertial force. This releases the bound teeth of the worm shaft and the worm wheel from each other in a favorable manner.